Pressure

What is Pressure?

To minimize your confusion, it would probably be best to avoid a
discussion of pressure altogether - but it's probably too late - you
are probably already confused on the issue of force and pressure!
Perhaps you think that force and pressure are really the same thing,
like mass and inertia.
Sorry!

Pressure depends on how much force is exerted - more force means
more pressure. In fact, pressure is directly proportional to force.
However, pressure is NOT the same thing as force.
Pressure also depends on the area over which the force is applied.
More area means less pressure - in fact, pressure is inversely
proportional to area:

Pressure Units:

The SI unit of pressure is the Pascal - a force of 1 Newton
applied over an area of 1 square meter produces a pressure of 1
Pascal, so a Pascal (abbreviated Pa) is a very small pressure.
Another common unit of pressure is pounds per square inch (psi).
Normal atmospheric pressure (1 atmosphere, abbreviated atm) = 14.7
psi = 1.0 x 105 Pa. Meteorologists use a unit of pressure
based on the height of a column of mercury that the pressure will
support, so on weather reports, you often hear atmospheric pressure
reported in "inches of mercury", where 1 atm = 29.9 inches of Hg.

The Difference Between Force and
Pressure:

When a force is exerted on an object, pressure is also exerted on
the object, but force and pressure are not the same thing. Here's a
"thought experiment" to illustrate the difference. Suppose that you
are going to exert a force of 5 Newtons on your lab partner, and
suppose your partner has the following choice:

A 5 Newton force exerted with the pointed-end of a pin.

A 5 Newton force exerted with the flat side of a Physics
book.

Assuming your lab partner is not VERY strange, which would
she pick? Of course, the book! But why? Both the pin and the book
will exert the same force, and she will get the same acceleration
from either force. Well, because the force exerted with the pin will
hurt - a lot - and the force exerted with the book won't.

Although both the pin and the book exert the same force, they
exert very different pressures. Pressure depends on the amount of
force and the area over which the force is applied. More force - more
pressure. More area - less pressure. In fact, pressure is directly
proportional to force, and inversely proportional to area.

Force tells you how an object will accelerate. Pressure
tells you how it will feel.

or:

Force accelerates. Pressure cuts.

Pressure Examples

Example 1: Why does a sharp knife cut better than a dull
knife?

The
diagram at right shows a sharp knife and a dull knife in contact with
a surface (maybe a nice, juicy steak!). You know that the sharp knife
cuts more efficiently - Why?

Notice that the sharp knife has a very small area of contact with
the surface, while the dull knife has a much larger area of contact.
If both knives are pushed down with the same force, the sharp knife
will exert a much greater pressure on the surface than the dull knife
- and pressure cuts.

So, if you are "stuck" with a dull knife, you have to exert much
more force in order to generate enough pressure to cut your steak,
right?

Example 2: Ice skates vs. shoes

Why
do people have to use ice skates? Why don't shoes work just as
well?

Look at the diagram on the right, which shows a shoe on the left
and a skate on the right. The downward force that you exert on the
ice in either case (assuming you are standing on two feet) would be
half of your weight.

Notice, however, that the shoe distributes the force over a much
larger area than the skate does. This means that the skate exerts a
much higher pressure on the ice than the shoe does - it is this high
pressure that makes ice skating possible!

Example 3: Why is a soft bed comfortable?

Why is it more comfortable (for most people!) to lie down on a
soft mattress than to lie down on the floor?

First of all, no matter what you lie on, it must exert an upward
(normal) force equal to your weight in order to support you.
Therefore, the soft mattress and the hard floor exert the same upward
force on you. However, the mattress conforms to your shape, so that
it exerts the support force over a larger area than the floor does.
Therefore, the mattress exerts less pressure on you than the floor.
Pressure hurts.